| 16 Mar 2026
Unraveling the chemical structures and sources of biomass-derived organic aerosols through a year-long offline analysis in Hyytiälä, Finland
Abstract. Biomass-burning OA (BBOA) and biogenic secondary OA (BSOA), both originating from biomass but from different pathways, still lack comprehensive and quantitative understanding, which limits assessments of their environmental impacts. In this study, source-resolved OAs including BBOA and BSOA in a European boreal forest were characterized by the offline use of an aerosol mass spectrometer (AMS), with improved chemical resolution offered by polarity-based fractionation. OA extract solutions were prepared according to polarity as high-polarity water-soluble organic matter, humic-like substances, and water-insoluble organic matter, and their abundances and chemical structures were analyzed by off-line high-resolution AMS analysis. Quantitative analysis revealed an annual OA concentration of 1.24 ± 0.75 µg m−3, with lower concentrations in winter and higher in summer. A 5-factor source apportionment solution was obtained from positive matrix factorization (PMF) of the mass spectra of the three fractions. CHN-family ions were found to be indicative of BBOA, whereas C5H8O5+, C5H6O+, and C8H9O4+ were identified as potential tracers for BSOA; they lead the identification of BBOA- and BSOA-like factors. CROA factor, related to aged fossil fuel combustion and aged biomass material combustion, was also identified. Different PMF factors exhibited differences in water solubility, with relatively water-insoluble characteristics of compounds containing CROA aromatic structures. This study highlights the usefulness of polarity-resolved factor analysis in understanding diverse OA sources and opens the door for the characterization of climate- and air-quality-related properties of BBOA and BSOA.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Sun et al. present a year-long offline HR-ToF-AMS study of polarity-resolved organic aerosol fractions from Hyytiälä, with PMF used to distinguish BBOA-like, BSOA-like, CROA, MO-OOA, and HOA components. The study is novel and potentially valuable because the polarity-based fractionation appears to provide additional chemical resolution beyond conventional online AMS-PMF approaches, particularly for separating biomass-burning and biogenic influences. The manuscript is generally well done, and I believe it is suitable for publication after minor revision.
My comments are mainly related to clarification, figure presentation, and several points where the interpretation could be made easier to follow.
The winter concentrations of the three OA fractions are quite small (HP-WSOM 0.13, HULIS 0.31, and WISOM 0.17 µg m⁻³). Please clarify the associated uncertainty for such low values, ideally in relative terms or in comparison with the uncertainty of total OA. This would help the reader assess the robustness of the seasonal interpretation.
The discussion of the PSCF results remains somewhat broad. The text states that air masses passed over Scandinavia and other parts of Europe, but from Fig. 3 the western-European influence does not appear equally clear for both events. Please specify more precisely which parts of Europe are implicated for P1 and P2. In addition, the map readability in Fig. 3 should be improved because the current figure is difficult to interpret.
Please carefully check the figure numbering throughout the manuscript and Supplement. In the Methods, all sample trajectories are said to be shown in Figure S4 (Lines 180–181), and the PMF stability analysis is also said to be shown in Figure S4 (Lines 193–194). Later, in the Results, the backward trajectory analysis is instead referred to as Fig. S6 (Line 242). This is confusing and should be corrected throughout the manuscript and SI.
Since earlier Hyytiälä studies identified SVOOA/LVOOA, it would be useful to briefly explain for readers how these previously reported factors relate to the MO-OOA terminology adopted here. This would make it easier to place the current results in the context of previous source-apportionment work at the site.
Please comment briefly on whether a six-factor or higher-factor solution was examined and why it was not retained. Also, there appears to be a typo in the description of the four-factor solution: it is written as “MO-OOA, BSOA-like factor, BSOA-like factor, and HOA,” where one of the repeated BSOA-like entries is presumably intended to be BBOA-like. The same duplication appears in the Fig. 4 caption (“MO-OOA, BSOA-like, BSOA-like, CROA, and HOA”). Please correct this in both the main text and figure caption.
The paragraph beginning with HOA is somewhat awkward because the section is focused on the BSOA-like factor. I suggest restructuring it so that the paragraph starts from the BSOA-like behavior and then introduces HOA as a secondary observation. As written, the logic is somewhat difficult to follow.
In the paragraph discussing the selected BSOA-related fragment ions, it would help the reader if the relevant figure reference were given at the start of the paragraph. At present, the text discusses the ions first and only later says that the results are shown in Fig. 5b. Since the previous paragraph already refers to Fig. 6, this section becomes harder to follow than necessary.
Since C₅H₆O⁺ is presented as a particularly source-specific tracer for the BSOA-like factor, please comment more explicitly on its seasonal behavior in the main text. That would further strengthen the interpretation.
When using the lodgepole pine AMS spectrum and discussing MBO emissions from coniferous trees such as pines, please clarify whether this is representative of the dominant tree species in the region surrounding the site.
P1 and P2 are introduced clearly in Fig. 2 and in the text, but it would help the reader if these periods were identified consistently throughout all relevant time-series figures, especially where their interpretation is discussed later.
The co-variation between nss-SO₄²⁻ and the BBOA-like factor is interesting, but the interpretation remains somewhat underdeveloped. Since the reported correlation is only moderate (r = 0.45), please discuss more explicitly what processes or source regions might explain this association and how selective this relationship really is relative to the other PMF factors.
Please check the formatting of m/z and ion notation throughout the manuscript for consistency.
The discussion of CROA is interesting. In addition to aged fossil-fuel emissions and aged biomass-combustion material, could the authors briefly comment on whether coal combustion may also contribute to this aromatic factor, or explain why that possibility is unlikely in this setting?
There are a few periods in the later polarity-distribution discussion that appear noteworthy, including elevated MO-OOA in the water-insoluble fraction in late July, the small winter increase in BSOA-like material, and the behavior of HP-WSOM during the same period. Please comment on whether these features reflect genuine compositional changes or could partly result from factor mixing/rotational ambiguity and larger uncertainty under low-concentration conditions.
Overall, I find the study novel and suitable for ACP after these mainly clarifying and presentation-related revisions.